The flow divider in Figure 13-15 is called a priority flow divider because it splits pump flow into a fixed controlled-flow (CF) outlet and sends excess fluid out an excess flow (EF) port. Volume orifices (drilled as specified by the purchaser) preset fluid flow out of the CF port. EF flow is any flow the pump produces over and above the controlled flow. This type flow divider is often used on vehicle power steering, where an engine-driven pump’s output may vary as rpm changes or as its flow is used for other functions. A priority flow divider assures that the power steering always has ample fluid at any engine speed or when other functions are active.

As fluid enters the valve, the path of least resistance leads through the controlled-flow-volume orifices and out port CF. If pump flow is more than the volume orifices can pass, pressure builds on the right end of the flow-control spool through the excess-flow pilot line. When pressure rises enough to overcome the bias spring and any backpressure from the steering circuit, the flow-control spool moves to the left, just enough to let excess flow exit through port EF. Excess flow changes as pump flow varies, but flow to port CF takes priority. A relief valve in port CF can be set for any pressure and has no affect on pressure at port EF. The controlled-flow relief valve is required even when maximum pressure is the same for both outlets.

Notice that controlled flow is pressure compensated. As pressure builds at port CF, it pushes back against the excess-flow pilot-pressure pilot to maintain a constant pressure drop across the volume orifices.

Priority flow dividers are also manufactured with adjustable flow for the priority port and without a relief valve for circuits that already have one. (The symbol shown is borrowed from a manufacturer's catalog because there is no standard symbol in ANSI or ISO literature.)

The flow divider in Figure 13-16 is a spool-type divider that splits flow at any predetermined rate according to the sizes of the drilled orifices. It is usually set up with identical orifice sizes for a 50-50 split. This particular design does not allow reverse flow, so bypass check valves are required when flow must return the same way it entered.

Fluid entering the Inlet port goes left and right through orifices, then out outlets 1 and 2. When either outlet encounters more backpressure than the other does, the high-pressure side forces the spool towards the low-pressure side until pressures on both sides equalize. Equal pressure drop across both orifices produces equal flow. (Most manufacturers specify flow equality at ±5%.) Pressure differences at the two outlets should be low because Inlet pressure always equals the highest outlet pressure -- which means pressure drop across the low-pressure outlet wastes energy.

Spool-type flow dividers only split flow. When more than two outlets are required, dividers must be used in series. A 50-50 split divider flowing into two more 50-50 dividers gives four equal outlets. A 66-33 divider into a 50-50 divider gives three equal outlets. The flow divider/combiner in Figure 13-17 equalizes flow in both directions. It can be used with double-acting actuators to synchronize speed in both directions of travel. The spool in this divider is made in two sections with a connecting link that allows the sections to move together in the closed condition (as shown) for combining, or be spread by Inlet pressure when they are dividing. Springs at both ends of the spool keep the sections together when pressure equalizes or is not present. Inlet orifices set nominal flow, while outlet orifices control flow to or from an actuator.

Flow to the inlet-return port goes through the inlet orifices to split into two equal parts. Pressure drop across the orifices causes the split spool to separate so the outlet orifices are working at the outer edge of the outlet-return ports. When unequal pressures on its ends shift the spool, flow is retarded to the low-pressure outlet port to keep it from receiving too much fluid. When the actuator reverses, flow into the outlet-return ports goes through the outlet orifices and on through the inlet orifices, causing the spool sections to come together. Now the outlet orifices control return flow on the inner edge of the outlet-return ports. They will retard flow from any actuator port that is trying to run ahead.